This invention relates generally to flexible circuits such as membrane probe cards and more particularly to the formation of electrical circuit conductors and connections thereon.
Testing integrated circuits at the wafer level is essential for the economic manufacture of these complex devices. By rejecting defective components at an early stage, unnecessary packaging costs are avoided. Also, wafer test data provides early feedback on the overall status of the IC fabrication process so that deviations can be quickly detected and corrected. VLSI technology places new demands on wafer test hardware as the technology increases in level of integration and operating speeds. A critical component limiting the performance of the test system is the wafer probe card.
A conventional probe card comprises a printed circuit board supporting an array of delicate, wire contact-styli which provide an electromechanical interface between a device under test (DUT) and the test electronics system. A major limitation of these probes is that the electrical environment of the interface is poorly controlled. Each wire stylus acts essentially as a lumped parasitic conductance of up to 20 nH. The result is to severely degrade signal fidelity at the DUT for frequencies above 50 MHz due to cross talk and high frequency attenuation. Also, these contact wires are so fragile that they do not maintain positional stability during service, so frequent maintenance and realignment are required for reliable operation. The alignment procedure becomes increasingly difficult with increasing pin count.
A membrane probe has been developed in an attempt to solve some of these problems. One of the earliest descriptions of a flexible probe was published in IBM Technical Disclosure Bulletin, Vol. 10, No. 10, March 1968, pages 166-67, entitled, "Film Supported Probe for the AC Pulse Testing of Integrated Circuits." This disclosure was elaborated in subsequent IBM Technical Disclosure Bulletins Vol. 13, No. 5, October 1970, pages 1263-64, entitled, "Membrane Type Probe" and Vol. 15, No. 5, October 1972, pages 1513, entitled "Flexible Contact Probe."
In 1988 International Test Conference IEEE Proceedings, pages 608-614, in a paper entitled, "Very High Density Probing," C. Barsotti et al. describe current probe card testing technologies including the use of a thin film hybrid diaphragm of controlled impedance signal, power and ground conductors aligned to traces on a supporting printed circuit board. This article describes present technology as limited to a four mil pitch signal line spacing, which is indicative of the limited permissible variation in width of line traces.
A current state-of-the art membrane probe is disclosed in "Membrane Probe Card Technology" by B. Leslie and F. Matta in 1988 International Test Conference, IEEE Proceedings, pages 601-607. The concept of the membrane probe is illustrated in FIG. 1. A flexible dielectric membrane supports a set of micro strip transmission lines that connect the test electronics to the DUT. Each transmission line is formed by a conductor trace patterned on one side of the dielectric membrane. A thin metal film on the opposite side acts as a common ground plane. The width of the trace is chosen to obtain a desired line impedance to match a particular device technology. Contact to the DUT, such as an individual integrated circuit die in a wafer, is made by an array of micro contact bumps formed at the ends of the transmission lines via holes in the membrane.
The overall structure of a membrane probe is shown in FIG. 2. The membrane of FIG. 1 is mounted on a printed circuit board carrier which interfaces with a test performance board via an appropriate connector. A forced delivery spring mechanism, supported on the carrier, pushes on the back surface of the membrane so as to protrude the contact bumps below the plane of the carrier. In operation, the membrane probe card is mounted on any commonly used prober and the wafers are stepped onto the probe in the same manner as with standard probes. Contact is made by raising the wafer toward the probe with a controlled overdrive. The spring system is designed to produce a uniform contact force over the entire array.
Although the membrane probe has been generally successful, a number of problems have been encountered in its fabrication and use. For efficient fabrication, it is desirable to use a common format or layout for the probe card for different purposes but, at the same time, it would be desirable to be able to customize the membrane probe.
One way that a membrane probe can be customized is to terminate selected transmission lines by shorting to the ground plane. This can be done by laser-drilling a hole through the dielectric membrane material and forming an electrically conductive VIA between the selected transmission line and the ground plane through the hole. The problem in doing this, however, is that it is difficult, particularly using conventional line-of-sight deposition techniques, to form a reliable conductor through such holes.
Various techniques are known for forming VIAs in printed circuit cards. U.S. Pat. No. 4,642,160 discloses a multi-layer printed circuit board manufacturing process. Metallic masks are formed on the surface of a layer of dielectric material, patterned to define VIA openings and irradiated from a laser light source to open VIAs in the dielectric into which an layer of copper is electrolessly deposited. EP Application No. 0 227 903 A2 discloses a method of etching through a metal layer on a metal/polymer layered structure using the technique of ablative photodecomposition (APD). APD relies on the use of ultraviolet laser radiation, which produces photochemical and other effects, as well as thermal effects, to remove irradiated material. The class of laser used for APD is commonly referred to as an excimer laser. IBM Technical Disclosure Bulletin, Vol. 29, No. 6, September 1986, pages 1862-64 describes how profiles of VIA holes in a polymer substrate can be varied from nearly vertical to very much tapered when a metal mask and excimer laser are used. P.E. Dyer et al. address the development and origin of conical structures in XeCl laser ablative polyimide in Appl. Phys. Let., 49 (8), Aug. 25, 1986, pages 453-55. Detailed discussions of ablative composition of polyimide and other polymer films appear in V. Srinivasan et al., "Excimer Laser Etching of Polymers," J.Appl. Phys. 59 (11) June 1986, pages 38, 61-67 and in J.H. Brannon et al., "Excimer Laser Etching of Polyimide," J. Appl. Phys. 58 (5) Sep. 1, 1985, pages 2036-43; B.J. Garrison et al., "Ablative Photodecomposition of Polymers," J. Vac. Sci. Technol. A 3(3) May/June 1985, pages 746-48; and J.R. Sheats, "Intensity-Dependent Photobleaching in Thin Polymer Films by Excimer Laser: Lithographic Implications," App. Phys. Lett. 44 (10), May 15, 1984, pages 1016-18. A paper by G.D. Poulin et al. entitled, "A Versatile Excimer Laser Processing System," SPIE Vol. 98, Excimer Beam Applications (1988), pages 17-23, describes the general state of the art to date of excimer laser processing of electrical circuitry on a polymer substrate. None of these references appears to suggest solutions to the problems discussed above.
Difficulties also arise in tailoring the transmission line impedance to each test situation. B. Leslie et al. disclose that this can only be done by varying the width of the transmission line trace. This measure is not adequate for all situations. Also, its use makes it harder to fabricate membrane probes with a common basic design. Different transmission line masks are required to vary the widths of the line traces. Varying line width is also inconsistent with obtaining the narrowest practical probe spacing, as is needed with increasingly dense integrated circuitry. It would be preferable to have a way to alter the transmission line impedance without varying line width or, to extend the range of possible impedance varying control, in combination with varying line width.
Accordingly, a need remains for an improved membrane probe card and method of fabrication of membrane probes.